Different molecules absorb different amounts of electromagnetic radiation depending upon the frequencies of the radiation. A particular molecule therefore has a unique absorption spectrum of absorption versus radiation frequency.
FIG. 1 shows a number of absorption lines of gaseous oxygen located in the range of approximately 760 nm to 770 nm at Standard Temperature and Pressure. The relative height of the various peaks represents the relative absorption of the oxygen. Taller peaks represent more absorption. Shorter peaks represent less absorption. The baseline represents little or no absorption.
As explained in U.S. Pat. No. 5,047,639 entitled "Concentration Detector" the concentration of a component of a material under test can be detected by identifying a frequency at which the component absorbs radiation and then passing radiation of that frequency through the material and detecting the amount of radiation that is absorbed. Because some molecules absorb radiation at different frequencies than do other molecules, it is often possible to isolate a characteristic absorption line of a particular component of interest in the material under test from the absorption lines of other components in the material under test. The detection of more radiation absorption indicates a higher concentration of the component of interest. The detection of less radiation absorption indicates a lower concentration of the component of interest.
In absorption measurements of gas concentrations using laser diode sources, the current supplied to the laser diode is typically dithered using a sinusoidal, triangular, or some other continuously cyclical perturbation of the laser diode current which causes wavelength modulation (WM) of the resulting laser radiation. Cyclical perturbation allows use of phase-sensitive harmonic detection methods both to center the modulation around the absorption line of interest and to measure the absorption strength. The first or third harmonic of the absorption measurement gives an asymmetric signal with a zero at line center useful for centering the radiation on the absorption line of interest. The second harmonic is symmetric about the absorption line thereby providing an amplitude which is proportional to absorption. Because the signal used to detect absorption occurs at twice the modulation frequency, the higher frequency absorption signal may be filtered from electrical and/or optical noise which occurs at the lower modulation frequency.
Another method of measuring absorption using laser diodes is called frequency-modulation (sometimes called FM spectroscopy). In FM spectroscopy, the modulation frequency is higher than the frequency half-width of the absorption line being measured. For narrow gas absorption lines this may require GHz modulation frequencies, but these high frequencies are free of excess low-frequency laser noise. By using two closely spaced modulation frequencies (two-tone FMS), signal processing at a more convenient MHz beat frequency is possible. As in wavelength modulation spectroscopy, phase-sensitive detection can be used. Little attention is usually given to the acquisition and maintenance of the absorption line at the center of the laser device modulation.
Another issue commonly discussed in the prior art literature is etalon effects which are created by windows and/or other parallel reflecting surfaces in the optical path. "Frequency Modulation and Wavelength Modulation Spectroscopies: Comparison of Experimental Methods Using a Lead-salt Diode Laser," by D. S. Bomse, A. C. Stanton and J. A. Silver, in Applied Optics, Vol. 31, No. 6, Feb. 20, 1992 and U.S. Pat. No. 4,934,816 entitled "Laser Absorption Detection Enhancing Apparatus and Method", issued to J. A. Silver and A. C. Stanton on Jun. 19, 1990 describe a fringe reduction technique where optical elements are vibrated mechanically at 30 Hz so that the fringes will be averaged by the high frequency and/or will be reduced by narrow band detection techniques.
Other sources of noise are also important. At low frequencies, randomly scattered light reentering the laser, drive current fluctuations, laser amplitude noise, laser frequency noise, detector noise, and amplifier noise are problems. The conventional solutions to these noise problems comprise the narrow band detection and frequency shifting techniques described above.